I'm currently designing a 4-layer PCB for a SOC (Allwinner A33 to be precise, 0.8mm-pitch BGA) with one DDR3 chip. Per DDR3 routing requirements, the traces need to have 50-ohm impedance. Also, they need to fit between BGA pads and breakout vias, which limits the trace width to max. 5mils.
To achieve 50-ohm impedance with a 5-mil microstrip, the dielectric layer needs to be about 3-4mil thick. Currently, I'm preparing to manufacture a few prototype pieces, and I'm having trouble finding a fabrication service online which would offer the required stackup.
So, the question is: how do people get such prototypes manufactured?